The Influence of W-DLC and Cr_xN Thin Film Coatings on Impact Damage between Bearing Materials

ABSTRACT

In this work, the effect of hard tribological coatings was studied in terms of mitigating impact damage between tungsten carbide spherical elements and two different types of steel substrates. The coatings included a hard, highly elastic Tungsten-incorporated diamond-like carbon (W-DLC) coating at two different thicknesses and a harder, less elastic Cr_xN coating. Impacts were created using a drop-test rig described herein and characterized in three ways: a measure of the coefficient of restitution during impact, investigation of the impact site using an optical interferometer, and fixed ion beam cross sections of select impacts for observation of subsurface damage within the coating and substrate. It was found that hard coatings on softer substrates such as 440C steel were able to mitigate surface damage up to a certain impact speed, depending on the coating, but were unable to influence the coefficient of restitution. On harder substrates like 52100 alloy steel, the coatings were found to increase the coefficient of restitution, indicating a reduction in energy loss due to plastic deformation, and to reduce damage at each tested speed. These effects and their potential influence on bearing performance are discussed in regard to impact mechanics, surface metrology, and the material properties of the coating and substrate acquired by nanoindentation.

KEYWORDS:

• Rolling element bearings
• solid lubricant coatings
• impact damage

Acknowledgements

The Timken Company is thanked for permission to publish. In particular, D. H. Smith and S. P. Johnson from Timken are acknowledged for facilitating the work and for their helpful discussions. B. Pendergrass is acknowledged for performing the FIB cuts and SEM imaging of dented regions.

Funding

This work was supported by a grant from The Timken Company, as well as the National Science Foundation's Graduate Research Fellowship.